1. Field of the Invention
The present invention relates to an ink composition suitably used for inkjet recording, an inkjet recording method and, furthermore, a lithographic printing plate obtained by employing the ink composition and a process for producing a lithographic printing plate. More particularly, it relates to an ink composition suitable for inkjet recording that cures with high sensitivity upon exposure to actinic radiation, and gives a cured material having sufficient flexibility even after the ink composition has been cured; an inkjet recording method; and a lithographic printing plate obtained by employing the ink composition and a process for producing a lithographic printing plate.
2. Description of the Related Art
With regard to an image recording method for forming an image on a recording medium such as paper based on an image data signal, there are an electrophotographic system, sublimation type and melt type thermal transfer systems, an inkjet system, etc. In the electrophotographic system, a process of forming an electrostatic latent image on a photosensitive drum by electrically charging and exposing is required, and the system is complicated; as a result, there is the problem that the production cost is high. With regard to the thermal transfer system, although the equipment is inexpensive, due to the use of an ink ribbon there is the problem that the running cost is high and waste material is generated.
On the other hand, with regard to the inkjet system, the equipment is inexpensive and, since an image is formed directly on a recording medium by discharging an ink composition only on a required image area, the ink composition can be used efficiently and the running cost is low. Furthermore, there is little noise and it is excellent as an image recording system.
With regard to an ink composition that can be cured by irradiation with actinic radiation such as ultraviolet rays (radiation curing type ink composition), as an ink composition for inkjet recording, for example, there is a desire for one that cures with high sensitivity and forms an image with high image quality. By achieving higher sensitivity, high curability upon exposure to actinic radiation can be imparted, and there are therefore provided various benefits such as a reduction in power consumption, longer lifetime of an actinic radiation generator due to a decrease in the load thereon and, as a result of adequate curing being achieved, suppression of evaporation of uncured low molecular weight material and of a reduction in the strength of an image formed. Furthermore, improvement in the image strength due to higher sensitivity imparts high plate life to an image when the ink composition is used for the formation of an image of a lithographic printing plate.
With regard to a UV curing type ink composition, for example, an ink composition employing in combination a plurality of monomers having different levels of functional groups has been proposed (ref. e.g. JP-A-5-214280; JP-A denotes a Japanese unexamined patent application publication). However, in such an ink composition, in order to maintain the curing speed, it is necessary to use a large amount of polyfunctional monomer, and in this case an image obtained after curing the ink composition has a problem with flexibility.
Conventionally, when a lithographic printing plate is produced, a so-called PS plate having a constitution in which a lipophilic photosensitive resin layer is provided on a hydrophilic support is used; this photosensitive resin layer is imagewise exposed to light to thus improve or degrade the solubility of the exposed area toward an alkaline developer and form an image, and the non-image area is then dissolved and removed. However, in recent years, a digitization technique of electronically processing, storing, and outputting image information using a computer has become widespread, and a new image output method that matches the above technique has been desired. In particular, a method that can produce a printing plate without a treatment employing a developer has been examined, and a process for directly producing a lithographic printing plate using an inkjet recording ink composition has been investigated. In this process, an ink composition is discharged imagewise on the surface of a preferably hydrophilic support by an inkjet method, etc., and this is then cured by irradiation with actinic radiation, thereby giving a printing plate having a desired image (preferably a hydrophobic image). In order to form an image area of a lithographic printing plate, it is desirable that ink composition droplets discharged onto a support cure quickly without spreading, the cured image area has excellent strength and adhesion to the support, and the image area follows flexure of the support well when the lithographic printing plate is set in a printer to thus prevent any occurrence of damage such as cracking, and there is currently a desire for an ink composition that is suitable for such an application.